1. Field of the Invention
The present invention relates to a projection cathode ray tube used for a color projection television set and so on.
2. Discussion of Background
The basic construction of a conventional projection display device using a cathode ray tube is such that an electron beam emitted from an electron-gun excites a fluorescent material in a phosphor screen having a relatively small dimension which is formed in the cathode ray tube, and thus obtained light emission on the phosphor screen is projected on a relatively large screen through an optical lens/disposed in the vicinity of and in front of the cathode ray tube to thereby obtain an enlarged picture image.
In such a projection cathode ray tube, a high light-output is required in comparison with a usual direct-view cathode ray tube in order to enlarge a picture image. For this reason, various techniques have been used. As an example, by utilizing the fact that light emitted from the phosphor screen has a cosine distribution in accordance with the so-called Rule of Lambert, the amount of light gathered by a lens can be increased by inclining a normal line on the phosphor screen to the optical axis of an optical system disposed in the vicinity of and in front of a panel. Accordingly, it is preferable that the inner curved surface of a panel portion has a part spherical surface or a curved surface of non-spherical shape protruded toward the electron-gun which emits the electron beam.
The optical system disposed in the vicinity of and in front of the cathode ray tube has a plurality of lens which are arranged in consideration of correction of aberration and other problems, in which a concave lens is disposed just before the cathode ray tube to effect correction of a curved image. However, when an angle formed between the screen and the electron-gun is large, it is impossible to effect a sufficient correction by the lens. Accordingly, it is effective that the fluorescent screen formed on the curved surface of the panel portion, when it is viewed from the electron-gun, is in a convex form as shown in FIG. 8.
FIG. 7 shows an example of the construction in combination of a conventional projection cathode ray tube having the panel portion with the inner surface of a convex form and an optical system. A glass bulb 1 is constituted by a funnel portion 3 having a large opening at one side and a small opening at the opposite side, a panel portion 2 sealingly attached to the large opening of the funnel portion 3 which has an inner surface 5 curved in a convex form in view from the direction of the electron-gun wherein a fluorescent screen 6 is formed on the inner surface 5, and a neck portion 4 receiving therein an electron-gun 40 which is sealingly attached to the small opening of the funnel portion 3.
The fluorescent screen 6 formed on the inner surface 5 curved in a convex form of the panel portion 2 emits light by striking on it an electron beam emitted from the electron-gun 40 received in the neck portion 4. Although a high light-output can be obtained by increasing an input power to the cathode ray tube, the input power undergoes restriction due to possibly breaking of the glass bulb and reduction in the life time of a fluorescent material due to heat generation, thermal characteristics of the projection lens, and so forth. In view of the problems, there is such a method that a structure for cooling a face 12 of the panel portion 2 by liquid to thereby realize a high light-output. Namely, a sealed chamber 34 is formed between a concave lens 21 constituting an optical system 20 having an optical axis 22 and the face 12 of the panel portion 2, and a liquid refrigerant 32 such as an aqueous solution of ethylene glycol having the same refractive index as glass is filled in the sealed chamber 34. In more detail, the sealed chamber 34 is defined by a flat surface of the concave lens 21 disposed at an end of a container 23 with a flange 24 in which a set of lenses is held against the face 12 of the panel portion 2 facing the flat surface of the convex lens 21 with a gap, a heat conductive bracket 31 made of aluminum with heat radiation fins 36, which is disposed between the convex lens 21 and the face 12 of the panel portion 2, which functions as a spacer, and pieces of adhesive 35 such as silicone resin disposed in a gap 45 between the heat conductive bracket 31 and the convex lens 21 and in a gap 46 between the face 12 of the panel portion 2 and the heat conductive bracket 31 to effect sealing of them. As shown in FIG. 9, the panel portion 2 is formed by filling molten glass 101 in a bottom mold 301, followed by pressing the molten glass 101 by a plunger mold 201, after which the glass is cooled for solidification. In consideration of making the operation of the plunger mold 201 easy and application of a stress load to the panel portion given by the atmospheric pressure, a side wall referred to as a skirt 8 is formed by retracting a sealing surface 9 from the inner surface 5 of the panel portion 2 toward the funnel portion 3. Generally, a useful screen area of the panel portion which can effectively display a picture image has a substantially rectangular shape in which the ratio of a vertical side to a lateral side is about 1:1-16:9. Accordingly, the outer contour of the sealing plane which is formed in the vicinity of the effective plane resembles that shape to thereby minimize the volume of the cathode ray tube.
In the cathode ray tube using the glass bulb having the above-mentioned construction, it is necessary that accuracy in the curvature of the inner surface of the panel portion is in an optically sufficient range in order to obtain the advantage of the panel portion having the curved portion in a convex form and good optical characteristics. Further, it is essential that the central axis of the optical lens formed by curving the inner surface of the panel portion in a convex form is optically in alignment with the optical axis of the optical system.
The inner surface of the panel portion which is formed by a method of pressing mold is apt to cause fluctuation of the curvature due to unstableness of the curved surface formed in a heated metal mold and unstableness of the behavior of glass under application of heat during cooling and solidifying of the glass. Further, inclination of the curved surface and deviation of the center of the curved surface may easily take place owing to errors in the metal molds, the pressing machine, and so on.
In order to eliminate such problems, the inner surface formed by press molding may be finished by mechanically polishing it to have a predetermined curved surface. However, since the skirt 8 and the sealing surface 9 extending toward the funnel portion 3 have a substantially rectangular contour as described before, it is impossible to polish entirely the effective plane. In this case, the area of the inner surface which does not contribute radiation of light greatly expands, and it is necessary to form the outer contour of the sealing surface 9 in a substantially circular shape rather than in a rectangular shape. Namely, in the construction of the conventional glass bulb, the volume of the cathode ray tube is greatly increased in order to increase accuracy of the curvature of the inner surface to be in an optically allowable range by mechanically polishing the inner surface.
In addition, when the cathode ray tube is once assembled, it is difficult to align the optical axis of the optical system with the central axis of the inner surface of the panel portion, and an inclination angle of the axis .DELTA..theta. and a deviation of the axis .delta. may be caused as shown in FIG. 7, which deteriorate the optical characteristics of the cathode ray tube.